Apparatus and method for characterizing movement of a mass within a defined space

ABSTRACT

Apparatus and methods for resolving presence, orientation and activity of a person within a defined space utilizes at least one electrode proximate to the space to be observed. An AC signal is applied to the electrode, and the current measured from that electrode and also to any other electrodes included in the system, and which are effectively connected to the ground return of the AC-coupled electrode. A person (or object) to be sensed intercepts a part of the electric field extending beween the AC-coupled &#34;sending&#34; electrode and the other &#34;receiving&#34; electrodes, the amount of the field intercepted depending on the size and orientation of the sensed person, whether or not the person provides a grounding path, and the geometry of the distributed electrodes. Given the nonlinear spatial dependence of the field, multiple electrodes can reliably distinguish among a set of expected cases. The invention can be configured to detect not only static positions and orientations, but also motion through a defined space. This is accomplished by taking sequential measurements at different times and utilizing the time variation in signal magnitudes as well as the absolute magnitudes themselves.

RELATED APPLICATION

This is a division of U.S. Ser. No. 08/606,540 filed Feb. 23, 1996, nowabandoned, which is a continuation-in-part of U.S. Ser. No. 08/191,042,filed Feb. 3, 1994, now abandoned.

FIELD OF THE INVENTION

The present invention relates generally to the sensing of position andthe distribution of mass within a spatial reference frame, and inparticular to a sensing system that resolves the presence, orientationand salient characteristics of a person in a defined space based onvariations in a displacement current.

BACKGROUND OF THE INVENTION

Determining the position, orientation or mere presence of a personwithin a defined space is important in applications ranging from medicaltreatment to safety and security. For example, prior to initiatingoperation of a tomographic scanning device, it is essential to ensurethat the patient is not only within the device, but also orientedproperly with respect to the scanning elements. While direct visualmonitoring by trained personnel is obviously ideal, the nature of theequipment may make this impossible.

Of even greater concern are devices that operate automatically but relyon an assumption of proper orientation. For example, airbags are nowemployed as standard equipment on new automobiles, and havesubstantially reduced accident-related injury by cushioning the driverand passenger. However, because of their explosive deployment, airbagscan themselves cause injury or even death of an infant improperlyoriented in the passenger seat. They are also expensive to repack, sothat passenger-side deployment into an empty seat is wasteful. Thus,although visual monitoring is simple and unimpeded in an automobile, itis not necessarily reliable; consumers may be unaware of proper airbagoperation, and the routine character of operating an automobile invitesinattentiveness: a driver might easily fail to disable the airbag afterhis passenger departs, or may orient a child improperly.

For such applications, where proper orientation is critical butdifficult to monitor visually with adequate reliability, sensor arrayshave been developed to obviate the need for human attention. Thesearrays typically contain a large number of sensing devices arranged tosurround the space of interest; the sensors are often utilized in abinary fashion--detecting or not detecting presence within an operatingrange--so that numerous devices typically are necessary to resolve thedifferent orientations of interest with a tolerable level of ambiguity.But the number of sensors is not chosen in any systematic or rigorousfashion; there is no methodology for minimizing the number of sensors,nor any test (other than experimentation) to ensure that the numberchosen will resolve all necessary cases.

In order to ensure reliability, current systems may also requiremultiple sensing modalities (e.g., some combination of infrared,ultrasound, force and capacitance sensing). Once again, the reason forthis stems from a "brute force" approach to distinguishing ambiguouscases.

For example, a common type of binary electrostatic sensor is acapacitive button switch, which is activated when the user places afinger thereon; in so doing, the user effectively increases thecapacitance of a capacitor, with the resulting increase in capacitivecurrent indicating actuation of the button. A more advanced version ofthis type of sensor is described in published PCT application WO90/16045 (Tait), which describes an array of receiver electrodesarranged about a central transmitting electrode. Even this type ofconfiguration, however, is only slightly more advanced than a purelybinary system, since what is measured is variation in weighting amongthe arrayed electrodes. Arrangements such as this do not meaningfullyreduce the number of devices (i.e., electrodes) necessary tocharacterize position and orientation, nor provide an approach toobtaining an optimum number of devices. Moreover, the Tait device is notemployed in a manner that is even capable of resolving athree-dimensional mass distribution, much less distinguishing amongdifferent orientation/position cases. It is expected that contact willbe complete in all cases--that is, the user's finger will actually touchthe transmitting and receiving electrodes--rendering the approachunsuitable where such contact is not possible.

DESCRIPTION OF THE INVENTION BRIEF SUMMARY OF THE INVENTION

The present invention dispenses entirely with the need for contactbetween the person under observation and the sensor, and substantiallyreduces the number of necessary sensing devices by multiplying thefunctions that each performs.

In accordance with the invention, a set of electrodes is arranged aboutthe space to be observed. An AC signal is applied to a first of theseelectrodes, and measurements taken of the current exiting that electrodeand also the currents into all of the other electrodes, which areeffectively connected to the ground return of the AC-coupled electrode.A person (or object) to be sensed intercepts a part of the electricfield extending beween the AC-coupled "sending" electrode and the other"receiving" electrodes, the amount of the field intercepted depending onthe size and orientation of the sensed person, whether or not the personprovides a grounding path, and the geometry of the distributedelectrodes.

For example, in a simple case, such as that contemplated in the Taitapplication, a person is so close to a sending/receiving electrode pairthat she effectively bridges the electrodes, increasing their capacitivecoupling and, therefore, the current through the receiving electrode ascompared with the current that would flow in the person's absence. If,however, the person is standing on the ground and is somewhat distantfrom the electrodes, her body provides a grounding path in addition toincreasing the capacitive coupling. These two effects are opposed: thepath to ground diverts some of the current, reducing the output at thereceiving electrode, while enhanced capacitive coupling increases outputcurrent. (The grounding path is insignificant in the simple case wherethe person touches both electrodes, since the capacitive-coupling effectpredominates.)

Also relevant is the absolute amount of current through the person,regardless of whether it reaches a receiving electrode or is shunted toground. Indeed, if the person is close to the sending electrode butdistant from the receiving electrodes, this will be the only relevantparameter.

All of these measurements, in combination with knowledge of electrodegeometry, help to resolve the position and orientation of the personwithin the space. Because the response of the field to mass distributionis a complex nonlinear function, adding electrodes can alwaysdistinguish among more cases. In other words, each electrode representsan independent weighting of the mass within the field; adding anelectrode provides information regarding that mass that is not redundantto the information provided by the other electrodes.

Yet perfect resolution is frequently unnecessary as a practical matter.The present invention is primarily intended not to fully resolveposition and orientation, but to distinguish among a discrete set ofallowed characteristics, positions and orientations. The geometry andnumber of electrodes are chosen so as to be capable of distinguishingamong these allowed possibilities with a desired degree of reliability.

To increase this reliability--that is, to reduce ambiguity--theinvention preferably includes means for switchably designating eachelectrode as either a transmitting or a receiving electrode. Making aset of measurements with the source and receivers located at differentpositions substantially increases the resolution capability of thesystem without increasing the number of sensors.

The invention also comprises a search methodology wherein sets ofmeasurements are represented as points in a multidimensional"measurement space"; with proper electrode orientation and a sufficientnumber of measurements in each set (i.e., a sufficient number of spatialdimensions), the spatial regions occupied by measurements taken atdifferent ones of the allowed positions and orientations will bediscrete and separable. In other words, despite some variation inposition and orientation within an allowed state, as well as inextraneous variables such as a person's mass, measurements will stilltend to cluster in spatial regions characteristic of the allowed states.

By maintaining a pattern characterizing the cluster regions and theirextents (generated, for example, by measuring different individuals inthe various allowed states), a new measurement can be unambiguouslyassociated with a cluster region and, therefore, a single one of theallowed states can be inferred.

The invention is usefully employed in any application wheredistinguishing among various positions and/or orientations, or thepresence or absence of an occupant, is important. Representativeapplications include airbag-deployment control systems; systems used tocontrol electrical devices (e.g., a reading chair with a lamp that turnson or brightens when a person is seated, or an office chair thatactuates a desktop computer when the chair becomes occupied); alertsystems (e.g., a seat for airplane pilots, automobile drivers and truckdrivers that issues an alert signal when the operator's head droops);intermittently active advertising systems (e.g., a multimedia kiosk orvideo-arcade game that senses passersby and signals them to attracttheir attention); and appliances or toys that "know" when they are beingheld.

The invention can be configured to detect not only static positions andorientations, but also motion through a defined space. This isaccomplished by taking sequential measurements at different times andmeasuring differences (if any) in the time-shifted signal magnitudes aswell as the absolute magnitudes themselves. In effect, time becomesanother measurement parameter, in the same way that adding an electrodeprovides a further spatial measurement parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention, when taken inconjunction with the accompanying drawings, in which:

FIG. 1A is a schematic diagram of a sensor incorporating the invention;

FIG. 1B is a schematic diagram of an alternative sensor design that canbe employed as a receiver or a transmitter;

FIG. 2 illustrates the manner in which a sensed object or person affectsthe parameters measured by the invention;

FIGS. 3A and 3B are elevational, partially schematic views of anapplication of the invention involving airbag deployment;

FIGS. 4A-4E schematically depict the measurement patterns correspondingto various different cases involving an automobile seat;

FIG. 5 illustrates the measurement space for three matrix positions andclusters formed by repeated measurements of the various casesillustrated in FIGS. 4A-4E;

FIG. 6A schematically illustrates a sensor configuration for recognizinggestures;

FIG. 6B depicts measurement patterns corresponding to various differentgestures as measured by the configuration shown in FIG. 6A; and

FIG. 7 illustrates the measurement space for the measurement patternsshown in FIG. 6B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer first to FIG. 1A, which illustrates a multi-electrode circuitsuitable for practice of the present invention. The circuit is arrangedto sense a characteristic of a mass 12 by detecting changes in theelectric field involving three electrodes variously employed asreceiving and transmitting electrodes as described below, it beingunderstood that the number of electrodes actually employed in a givenimplementation depends on the nature of the application. In therepresentative mode of operation shown in the figure, the electrodesinclude a sending or transmitting electrode T and a pair of receivingelectrodes R₁, R₂. The object 12 may be, for example, a human body orportion thereof; the characteristic to be sensed depends on the relativepositions of the three electrodes (with respect to each other and toobject 12), and the manner in which they are used.

The circuit includes components defining a transmitter stage, switchablycoupled to transmitting electrode T, and a pair of receiver stagesswitchably coupled to receiving electrodes R₁, R₂. The transmissionstage includes an alternating-current (AC) source 18 connected, by meansof a switching logic circuit 20, between the electrode T and a referencepoint, i.e., ground, with a shielded cable 22 being used for theconnections between source 18 and electrode T. Electrodes R₁, R₂ arealso connected to the output side of switch logic 20 (by means ofshielded cable 19), and the other two inputs to switch logic 20originate with a pair of receiver stages switchably connected toelectrodes R₁, R₂. Each receiver stage includes an operational amplifier25₁, 25₂ connected in a negative feedback circuit. Thus, each of the tworeceiver input terminals is connected to the inverting input terminal ofone of the amplifiers 25₁, 25₂. The inverting input terminals are thusessentially at ground potential and the output voltage of each of theamplifiers corresponds to the current from electrode T to ground.

A resistor 27₁, 27₂ and a capacitor 28₁, 28₂ bridge the non-invertinginput terminal and the output terminal of each amplifier 25₁, 25₂, whichare, in turn, each connected to a synchronous detector 29₁, 29₂, whoseother input is a signal from source 18. Accordingly, the output of thedetectors 29₁, 29₂ is the component in the output of amplifier 25₁, 25₂that has the frequency and phase of the source 18. It is thus free ofinterfering signals and noise that may be picked up by the electrodesR₁, R₂.

The receiving stages each also include a low pass filter 31₁, 31₂ whichsmooth the output of detectors 29₁, 29₂. The signals from filters 31₁,31₂ are applied to a computer processor 33, which includes ananalog-to-digital converter (not shown) that converts the voltage fromthe filter to a digital value. The computer 33 controls switch logic 10,and utilizes the signals from filters 31₁, 31₂, as described below.

A resistor 34 is connected between the output of source 18 and one inputterminal of a voltage detector 36, the other input terminal beingconnected directly to the output of source 18. In this way, detector 36can be calibrated to measure the current output of source 18, and itsoutput is provided to computer 33.

The frequency f₁ of source 18 may be 100 kHz, and the relative spacingof the electrodes depends on the characteristics being sensed. In anycase, the length of the electrodes and the spacing between them aresubstantially less than a wavelength at the frequency f₁. Accordingly,there is minimal radiation from electrode T and the coupling toelectrodes R₁, R₂ is essentially capacitive.

FIG. 1B illustrates an alternative arrangement that avoids the need forswitch logic and separate receiver and transmitter stages. The circuitincludes an electrode E, which can be a transmitting or receivingelectrode; an AC source 18; a transimpedance amplifier 41 measuringcurrent; a differential amplifier 43; a synchronous detector 29 whoseinput terminals are connected to the output of amplifier stage 43 andsource 18; and a low-pass filter stage 46. A switch 48, controlled bycomputer 33, determines whether electrode E is a transmitting orreceiving electrode.

In operation, closing switch 48 applies the signal from AC source 18 toelectrode E (via feedback) causing the circuit to operate as atransmitter. The input to the second gain stage 43 is a voltageproportional to the current into electrode E, so the ultimate signalreaching computer 33 reflects a loading measurement.

Opening switch 48 decouples electrode E (but not detector 29) from ACsource 18, so the signal into electrode E, which the other circuitcomponents amplify and filter, originates externally (i.e., with asimilar circuit behaving as a transmitter). The introduction of a person(or a person's limb or feature) into the electric field extendingbetween, e.g., electrode T and electrodes R₁, R₂ may be understood withreference to FIG. 2, which models the various interactions in terms ofan equivalent, hypothetical circuit. In the figure, the person P isrepresented as a three-terminal network, and current from AC source 18(transmitted through a transmitting electrode T) can reach ground viaany of three current paths: through a first variable capacitor 50connected to a receiving electrode R (spaced some distance fromelectrode T and connected to ground); through a pair of variablecapacitors 52, 54 on either side of network P and then to electrode R;or directly through network P and a fourth variable capacitor 56. Thevalues of the various hypothetical capacitors in the circuit depends onthe relative distances between the electrodes R, T and the person P, andthe circuit assumes that person P is positioned within the space definedby the electrodes.

The capacitor 50 represents capacitive coupling solely between theelectrodes, as if they were the two plates of a single capacitor.Without the person P, this capacitance would predominate; whenintroduced, however, person P, who is electrically conductive, "steals"flux from the electric field between the two electrodes and conducts itto ground via capacitor 56, but also increases the capacitive couplingbetween the electrodes by changing both the effective geometry and thedielectric constant; this increase in capacitive coupling is representedby the capacitors 52, 54.

In "loading" mode, current into receiver electrode R is inconsequentialor ignored for measurement purposes; the only relevant current is thatexiting electrode T, regardless of how it reaches ground. For example,if electrode R is very far from both electrode T and person P, which areproximate to one another, the dominant capacitances will be those at 52,56, and the current exiting T--as measured, in FIG. 1A, by detector36--will essentially equal the current through person P.

If, however, the electrodes are spaced more closely, the electric fieldbetween them is stronger, and the other capacitances become moresignificant; their relative significances, of course, depend on thelength scale and position of person P with respect to the electrodes. Ifperson P is very close to electrode T, the person's body is effectivelyclamped to the AC source, so it oscillates at the transmission voltage.In this "transmit" mode, capacitance 52 is therefore very large relativeto capacitances 54, 56. Because AC source 18 is configured to deliver aconstant voltage, the increase in capacitance 52 as person P approacheselectrode T forces AC source 18 to put out more current (which can bedetected by a "loading mode" measurement) in order to maintain theconstant voltage. This results in greater current at electrode R, theamount of the increase depending on the ratio of capacitance 56 tocapacitance 54 (the magnitude of capacitance 54, in turn, depending onthe distance between the person P and electrode R).

When there is some distance between person P and both electrodes,capacitance 56 is overwhelmed neither by capacitance 52 nor capacitance54, and therefore contributes to the current detected. In this "shunt"mode, some of the field is shunted to ground, and the effect ofcapacitance 56 is to reduce the current at electrode R. The shuntedcurrent is maximized when the person is situated halfway betweenelectrodes T and R, since capacitances 52, 54 are thereby minimized (andcapacitance 56 is assumed not to vary significantly with position); ifthe person P moves closer to either electrode, one of capacitances 52and 54 will increase and the other will decrease, but the net effect isgreater current into electrode R. Naturally, the shunting effect isincreased to the extent person P's coupling to ground is improved (thelimiting case occurring, for example, when person P touches a groundedwire).

These three cases represent the most extreme situations that may beencountered, and are therefore most easily interpreted in terms ofsignal measurements. For example, a high current out of electrode T andvirtually no current into electrode R unambiguously locates person Pproximate to or touching electrode T and far from electrode R. But mostcases are intermediate, resulting in degeneracy. That is, the signalvalues cannot unambiguously characterize the location and orientation ofperson P, because those values can be produced by more than one uniquelocation and orientation.

Rather than attempt to resolve degeneracies by increasing the number ofelectrodes, the present invention increases the number of measurements.By selectively connecting the AC source 18 to different ones of a set ofelectrodes, and measuring the current exiting the AC-coupled electrodeand into the other electrodes, a matrix of measurements can be obtained.If each of n electrodes is employed as a transmitting electrode withcurrent readings taken both from the transmitting and other electrodes,the matrix is square ##EQU1## for i=j=n electrodes. The diagonal termsm₁₁, . . . m_(ij) refer to measurements made in loading mode, i.e., thecurrent out of the transmitting electrode; the entry m₂₁ refers to thecurrent into electrode 2 when electrode 1 is the transmitter; and theentry m₁₂ refers to the current into electrode 1 when electrode 2 is thetransmitter.

Even if this measurement matrix is insufficient to fully characterizethe location and orientation of person P, it can still be used to placethe person within a defined range of possible locations and orientationsbased on known, characteristic measurement patterns. For example,suppose the person is constrained to a fixed number of orientationswithin the measured field. While different people in slight variationsof the allowed orientations will produce different measurement patterns,these patterns will still tend to cluster. So long as the differentclusters can be mathematically resolved and are distinct from oneanother--that is, the electrode arrangement is judiciously chosen toavoid measurement degeneracy among allowed orientations--newmeasurements can broadly be said to belong to one of the clusters and anorientation among the allowed orientations thereby assigned.

To characterize these clusters, one can, for n electrodes, define ameasurement space of n² dimensions, with each matrix of measurements fora particular person in a given orientation constituting a single pointin this space. It is these points that will tend to cluster within agiven spatial region for each allowed orientation, and so long as thecluster regions are separable (i.e., classifiable by decision boundariesin the measurement space), it will be possible to unambiguously inferorientation by locating the cluster to which a new measurement belongs.

Importantly, because each additional electrode represents a weighting ofthe mass in the field that is independent (due to the nonlinearity ofthe response of the field to mass distribution), n electrodes providesenough information to distinguish among roughly n² different cases (then² figure being limited only by symmetry and insufficient measurementvariation among the different cases to permit their reliableresolution). Thus, adding even one electrode substantially increases thenumber of cases that can be resolved. As a practical matter, this meansthat an initial configuration capable of distinguishing among many casesand failing only for a few can usually be extended to resolve theambiguous cases through addition of a single electrode.

The manner in which the invention is practiced may be understood withrenewed reference to FIGS. 1A and 1B. Computer 33 is associated with amemory 33m that stores both data and executable programminginstructions. In the simplest approach, these instructions causecomputer 33 to operate switch logic 20 so as to couple AC source 18sequentially to the three illustrated electrodes. Each time source 18 isso coupled, computer 33 reads signals from filters 31₁, 31₂ and fromdetector 36, thereby making a row of measurements, and stores these inmemory 33m. Memory 33m also includes pattern information concerningcluster locations in n² space. This information can consist simply oflarge numbers of previous measurements which, when represented as pointsin n² space, naturally cluster in characteristic regions. Preferably,however, it includes a more precise mathematical characterization (e.g.,in the form of templates) of the cluster patterns and their boundaries,which thereby serve as decision boundaries. These boundaries aredetermined using linear or nonlinear clustering algorithms well known tothose of skill in the art. So long as the boundaries are discrete andseparable, assessing the location in n² -space of a new n² -dimensionalmeasurement most likely places it within a boundary, unambiguouslydesignating the characteristics associated with that bounded region; orplaces it outside a boundary, indicating that the characteristics do notmatch any of the known (or allowed) cases upon which the boundaries arebased.

Obviously, memory 33m may be configured to include permanent storagecontaining several different cluster patterns corresponding to differentallowed orientation sets or electrode arrangements, with computer 33including a suitable user interface to facilitate selection thereamong.

Depending on the cluster distribution, it may not be necessary tocomplete the full matrix of measurements in order to characterizepresence or orientation. The system can be designed along a hierarchy,proceeding only as far as is necessary to characterize the parameter ofinterest. As a trivial example, a single loading-mode measurement may besufficient to resolve presence; more generally, the followingmeasurement hierarchy is useful in the design of a variety ofimplementations:

1. Loading-mode measurement with a single electrode;

2. Two electrodes, one a fixed transmitter, the other a fixed receiver;

3. Two electrodes each switchably operable as a transmitter or areceiver, facilitating a full 2×2 measurement matrix;

4. Add further electrodes and measurements according to guidelines 1-3.

As is obvious from the measurement matrix, the different measurementmodes are not mutually exclusive; instead, they each contributedifferent information toward characterizing the mass and distinguishingamong cases. For example, "loading mode" measurements are by no meansuseful only where current into receiving electrodes is negligible;instead, they are utilized in conjunction with other measurements thatare by themselves inadequate; each measurement contributes independent,non-redundant information. (For ease of presentation, the terms "shuntmode" and "transmit mode" are sometimes used herein to designate thedominant mode of a particular measurement, rather than in an exclusivesense.)

Another strategy for resolving degeneracy is the use of multipletransmission frequencies, since the output voltage of the sensor 10 is afunction of the frequency f₁ of source 18 in addition to theconfiguration and spacing of the various electrodes and the orientationof the person with respect thereto. Multiple AC sources, each operatingat a characteristic frequency, can simultaneously operate through asingle transmitting electrode; or, instead, each of the electrodes, whenused as a transmitting electrode, can be energized at one or morefrequencies different from those at which the other electrodes operate.

Thus, again with reference to FIG. 1A, the electrode T can be connectedto receive signals from both the source 18 and a second source 18_(a)having a frequency f₂. The sources 18 and 18_(a) are coupled to thereceiving electrodes R₁, R₂ through isolation filters 40 and 40_(a),tuned to the frequencies f₁ and f₂, respectively. The output of theamplifier 20 is applied to a second synchronous demodulator 29_(1a)connected to the source 18_(a). The output of the demodulator 22_(1a) ispassed through a low-pass filter 24_(1a), whose output in turn is fed tothe processor 33. Since the output currents from the electrodes R₁, R₂are, in part, a function of frequency, the use of multiple frequencysources provides, in essence, multiple sending and receiving electrodessharing common physical electrodes.

A highly advantageous application of the present invention is detectionof presence and orientation in automobile seats designed for infants andsmall children. The need for this type of measurement has taken onspecial urgency with the introduction into automobiles of airbag safetydevices, which inflate explosively upon the automobile's impact. If thechild seat is installed in the front passenger side of the automobile,it is critical to the safety of the infant that the seat 75 facebackward, as shown in FIG. 3B, opposing the back rest 77 of theautomobile seat 79; in this way, should the automobile deceleraterapidly, the infant's head will be supported by the upright portion ofthe seat and not snap forward. However, the infant's head is then indangerously close proximity to the explosive airbag. In thisorientation, the airbag must be disabled.

If, on the other hand, the child seat 75 is oriented forward, toward thewindshield as shown in FIG. 3A, the explosive deployment of the airbagis not a threat because the seat retains the infant an adequate distancefrom the airbag.

At the same time, replacement and repackaging of the airbag followingdeployment entails considerable expense. Accordingly, the airbag shoulddeploy only if the passenger seat is occupied, and if occupied by aninfant, only if the child seat is oriented such that the infant facesforward.

All of the relevant possibilities can be resolved using four electrodes82, 84, 86, 88 as shown in FIGS. 3A and 3B; in fact, for thisapplication reliable results can be obtained with only three electrodes,as discussed further below, so that either of electrodes 84, 86 (bothshown in phantom) can be omitted from the design. As shown in FIG. 3B,the electrodes are connected to the circuitry and processor arrangementshown in FIG. 1A or 1B (and indicated generically at 95); the processor,in turn, is connected to a circuit 97 that controls operation of theautomobile airbag.

Refer now to FIGS. 4A-4E, which descriptively illustrate the effects ofvarious cases on the 3×3 measurement matrix obtainable with threeelectrodes generically labeled E₁, E₂, E₃ that may be switchablyemployed as transmitting or receiving electrodes in accordance withFIGS. 1A and 1B. The coarsest relevant measurement is whether the seatis occupied by a person at all. This measurement is not trivial, sincethe driver may load groceries or other items onto the seat, and anyuseful sensor array must be capable of distinguishing persons frominanimate objects. Although food or merchandise tend to exhibitconductivities different from that of a person (plastic andglass-packaged items will tend to be poor conductors, for example, whilemetal items conduct much better), the average or aggregate conductivitymay not differ significantly. On the other hand, the conductivitydistribution probably will not match that of a person, so that a fullmatrix of measurements will easily distinguish person from object.

Thus, in FIG. 4A, seat 77 is empty. Table 100_(a) represents themeasurement matrix, with the height of each tabular entry graphicallydepicting the amount of current into a receiving electrode. The sensorshave all been calibrated so that all current levels are the same andscaled to the halfway point.

By contrast, because an adult spans all three electrodes, as shown inFIG. 4B, transmission-mode coupling is characteristically large andrelatively uniform through the measurement matrix as illustrated intable 100_(b). The effect is similar but diminished in the case of aforward-facing infant, as shown in FIG. 4D. Transmission-mode couplingbetween all electrodes, while substantial and uniform, is not as largeas in the case of the adult.

In the case of a rearwardly oriented infant, as shown in FIG. 4C,transmission-mode coupling between electrodes E₃ and the other twoelectrodes is quite small, while coupling between electrodes E₁ and E₂is similar to that produced by a forward-facing infant.

The measurement pattern produced by an object such as a bag ofgroceries, as shown in FIG. 4E, is easily distinguished from the others.It is explained by the fact that when the electrode beneath thegroceries (i.e., electrode E₁) is the transmitter, it puts out arelatively large current, resulting in large loading-mode andtransmit-mode readings; when either of the other electrodes transmits,the effect of the groceries is insubstantial.

These various cases produce measurement clusters that are separable andreadily resolved, as illustrated in FIG. 5. For ease of presentation athree-dimensional measurement space is shown, reflecting measurementsm₂₁, m₃₁, m₃₂ only; obviously, the 9-dimensional space incorporating allmeasurements would produce even better results (i.e., greaterseparability among measurement clusters). A first axis 110 plots m₂₁(transmitter=E₁, receiver=E₂); a second orthogonal axis 112 plots m₃₁(transmitter=E₁, receiver=E₃); and a third orthogonal axis 114 plots m₃₂(transmitter=E₂, receiver=E₃).

Multiple measurements for each of the various cases are represented bypoints, which cluster as illustrated; each point represents a separateset of m₂₁, m₃₁, m₃₂ measurements, and was generated to simulate anactual case by perturbing a representative measurement point with noise.The cluster 120_(a) corresponds to variations of the measurements shownin table 100_(a), the cluster 120_(b) corresponds to table 100_(b), andso on. The three-dimensional construct 125 depicts the decision boundarybetween the clusters, which are discrete and well-separated.

In another embodiment, the invention is configured to resolve gesturesby plotting redundant measurements separated in time. In other words,what is measured is not only the magnitudes of the transmitted and/orreceived current, but also their changed values at a different time. Theoriginal and "lagged" measurements represent separate parameters forpurposes of measurement space.

A representative configuration appears in FIG. 6A, where, for purposesof simplicity, the transmitter T and the receiving electrodes R₁, R₂ arefixed; this is ordinarily desirable, since the mass to be measured is inmotion, but in principle there is no reason why transmitting/receivingidentities cannot be exchanged among electrodes. A hand H is free tomove left, right, up and down relative to the electrode array, and themeasurements taken, as shown in FIG. 6B, include essentiallysimultaneous measurements at R₁ and R₂, and a previous or "lagging"measurement R₁ '. In general, the simultaneous measurements at R₁ and R₂are taken periodically, and R₁ ' is retained in memory 33m from theprevious measurement set.

In FIG. 6B, the illustrated position of the hand H is moved (R)ight,(L)eft, (U)p or (D)own from an initial position, and the resultsgraphically depicted in table 130. Leftward movement begins with hand Hin the illustrated position; shunt-mode coupling to R₁ is initiallysmall, as illustrated by the large R₁ ' measurement, but increases whenthe hand has moved to a point (not illustrated) between R₁ and R₂ (asshown by the R₁ and R₂ measurements). The upward and downward movementsassume that the hand is situated over the T electrode.

Because the electrodes do not change in function, three measurements aretaken for each case, and therefore can be represented by athree-dimensional measurement space as shown in FIG. 7. The space ischaracterized by orthogonal axes 132 (corresponding to measurements atR₁), 134 (corresponding to time-lagged measurements at R₁), 136(corresponding to measurements at R₂). Numerous instances of the fourcases shown in table 130, each perturbed with noise to simulate slightvariations, cluster into four distinct regions corresponding to upwardmovement (cluster 140_(U)), downward movement (cluster 140_(D)),leftward movement (cluster 140_(L)) and rightward movement (cluster140_(R)) as described above.

It should be understood that the foregoing configuration isrepresentative only; as few as one electrode (multiple time-separatedloading-mode measurements) or two electrodes can resolve numerousmovement modes as well.

It will therefore be seen that the foregoing represents a highlyextensible and reliable approach to characterizing presence, positionand orientation within a defined space. The terms and expressionsemployed herein are used as terms of description and not of limitation,and there is no intention, in the use of such terms and expressions, ofexcluding any equivalents of the features shown and described orportions thereof, but it is recognized that various modifications arepossible within the scope of the invention claimed.

What is claimed is:
 1. Apparatus for characterizing movement of a masswithin a defined space, the apparatus comprising:a. a plurality ofelectrodes proximate to the space, the electrodes having fixed positionsrelative to each other; b. an AC source; c. means for momentarilyconnecting at least a first electrode to the AC source a plurality oftimes so as to produce a plurality of electric fields; d. means formeasuring current into the unconnected electrodes each time thefield-producing electrode is connected to the AC source, therebyproducing a plurality of measurements through the unconnectedelectrodes, at least some of the electrodes being switchably connectableto the AC source or to the current-measuring means to thereby serve asfield-producing electrodes or as measurement electrodes; and e. meansfor characterizing movement of the mass within the space based on theplurality of measurements.
 2. The apparatus of claim 1 comprising atleast three electrodes, the connection means being configured to connecta first of the electrodes to the AC source, and the measurement meansbeing configured to make at least two measurements of current into asecond of the electrodes and at least one measurement of current into athird of the electrodes.
 3. The apparatus of claim 1 comprising at leastthree electrodes, the connection means being configured to sequentiallyconnect each of the electrodes to the AC source, and the measurementmeans being configured to make at least two measurements of current intoone of the unconnected electrodes and at least one measurement ofcurrent into another of the unconnected electrodes.
 4. The apparatus ofclaim 1 further comprising:a. a computer memory for storing the (i) themeasurements and (ii) a pattern of measurement clusters each associatedwith a movement; and b. a processor for comparing the measurementsagainst the pattern to thereby characterize movement of the mass.
 5. Amethod of characterizing movement of a mass within a defined space, themethod comprising the steps of:a. providing at least two electrodesproximate to the space; b. sequentially sending an AC signal out each ofthe at least two electrodes at fixed intervals so as to produce aplurality of electric fields; c. measuring current out of the electrodeas the AC is sent; and d. characterizing movement of the mass within thespace based on the measurements by (i) providing a pattern ofmeasurement clusters each associated with a movement, (ii) comparing themeasurements against the pattern, and (iii) based on the comparison,characterizing the movement.
 6. A method of characterizing movement of amass within a defined space, the method comprising the steps of:a.providing at least two electrodes proximate to the space, the electrodeshaving fixed positions relative to each other; b. momentarily sending anAC signal out a first electrode a plurality of times so as to produce aplurality of electric fields; c. measuring current into at least oneunconnected electrode each time the signal is sent out the firstelectrode, thereby producing a plurality of measurements; and d.characterizing movement of the mass within the space based on theplurality of measurements by (i) providing a pattern of measurementclusters each associated with a movement, (ii) comparing themeasurements against the pattern to thereby characterize movement, and(iii) based on the comparison, characterizing the movement.
 7. Themethod of claim 5 wherein the mass is a person.
 8. The method of claim 6wherein the mass is a person.